Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a die saw, and then packaged into individual components.
During the packaging process, several semiconductor die are attached to a lead frame, often with a material such as epoxy or other viscous adhesives. Bond wires couple each of several bond pads on each die to conductive leads on the lead frame. The die, the wires, and a portion of the leads are encapsulated in plastic or encased in ceramic. These leads couple the die with the device into which the component is installed, thereby forming a means of I/O between the die and the device.
One step of semiconductor manufacture that is not without problems is the die-lead frame attachment. During the manufacturing process, several die are attached to the lead frame, wires are connected from the bond pads on each of the die to the "fingers" on the lead frame, then the die is encapsulated in a protective plastic casing. The plastic packages are separated, and the leads are formed into a desired shape. The lead frame, part of which will eventually form the conductive leads of the components, contains a major surface to which the die is attached, called the "paddle." The die is normally bonded to the paddle with epoxy or another viscous adhesive, although thermoplastic, tape, or another materials are also used. FIG. 1 shows a lead frame 10 having die paddles 12 with die 14 attached. The adhesive used to attach the die to the lead frame is dispensed on the die paddle area of the lead frame. The die is placed on the uncured epoxy and held at a specific pressure by die attachment equipment having a surface contact tool or an edge contact only tool (collet). The die is pressed down into the adhesive at a specific pressure by the tool and held in place long enough to ensure adhesion. X-Y movement (scrub) is sometimes used to increase adhesion and to speed the process. The attach process requires a follow-on cure in a separate cure oven.
Various problems are associated with the connection of the die to the die paddle. Occasionally a corner of the die will crack, thereby making the semiconductor inoperable. This can result from stress placed upon the die by the adhesive due to an uneven thermal coefficient of expansion between the die and the adhesive. After the die is attached to the lead frame and oven cured, the assembly is heated at the wire bond step to attach the wire to the die pad. If the die and the adhesive expand at different rates, undue stresses can be inflicted on the die thereby causing the corner to crack. Corner crack can also occur from stress on the die due to shrinkage of the adhesive as it cures, although in recent years chemical improvements in adhesive has reduced this cause of corner crack.
Occasionally the die and epoxy may come loose from the lead frame, a problem referred to as "popping die." Popping die can result from too little adhesive under the die, a poor bond between the adhesive and the paddle, or from bowing of the die paddle from heat or pressure. This can be a serious problem, not only because it results in scrapping the die but also because the loose die can damage the molds which are used to encapsulate the package. A single popping die can presently cost as much as $11,000 in damaged equipment.
Many of the problems associated with the die-lead frame attachment are thought to result to varying degrees from package stresses caused by nonuniformity in the thickness (the "bond line") of the adhesive which bonds the die to the lead frame paddle. This thickness has been controlled mainly by dispensing a measured quantity of adhesive then applying a controlled pressure for a specific time to the die by the die attacher. The bond line is difficult to control in this manner, and can vary greatly with small variations in the viscosity of the adhesive, application temperature, amount of applied adhesive, and other factors. If not enough adhesive is applied to the die paddle or too little pressure is applied to the die, a good bond between the die and lead frame cannot be ensured because of voids in the adhesive under the die. If too much adhesive or pressure is applied, the bond may be strong but too much of the adhesive will ooze out from under the die (resin bleed) and can prevent a good coupling between the bond wire and the bond pad on the top surface of the die. "Wet out" refers to the adhesive which seeps out from under the die, and the "fillet height" is the height reached by the adhesive which seeps out. A good bond will have 100% wet out but a very low fillet height, which helps prevent resin bleed and popping die from too little of a bond line. FIG. 2 shows a die 14 attached to a lead frame 10 by an adhesive 20, with a good wet out and fillet 22. FIG. 3 shows a die 14 attached to a lead frame 10 by an adhesive 20, with a poor wet out (voids 30 beneath the die 14) and a fillet 22 which covers the bond pads (not shown) on the top surface of the die 14.
Other methods of controlling the thickness of the adhesive under the die have been tested, with limited success. Manufacturers have attempted to measure the fillet height of the adhesive that oozes out from beneath the die as pressure is applied to the die and cease applying pressure to the die as soon as the fillet reaches a certain height. Adhesives with solid suspended "microbeads" have been manufactured in the attempt to control the bond line. Lead frames have been manufactured with dimples which collect excess adhesive as pressure is applied and therefore helps relieve stress which might otherwise bow the package.
A reliable method of controlling the bond line thickness which would help reduce or eliminate the various problems describe above would reward chip manufacturers reduced costs from scrap and damaged equipment.